R-type virus-like particles (R-VLP) are a morphologically unique type of VLP which are found in normal and malignant hamster cells and in some continuous lines of hamster cells, including the widely used BHK21 clone 13 line of baby hamster kidney fibroblasts. The R-VLP differs morphologically and in intracellular site of replication from A-, B- and C-type oncorna viruses. Like the oncorna viruses, however, R-VLP virions contain high molecular weight, 60 to 70 s, single stranded RNA. Some RNA directed DNA polymerase activity has been detected in microsomal fractions of hamster melanoma cells which contain large numbers of R-VLP. Therefore, the RVLP may replicate through provirus DNA as do oncorna viruses. There is evidence which suggests that R-VLP may be the etiological agents of malignant melanoma and an ascites tumors in hamsters. We will utilize the sensitive technique of nucleic acid hybridization to determine whether the replication of the RNA-containing R-VLP involves a DNA intermediate state. The nucleic acid sequence homology of R-VLP with hamster leukemia and sarcoma virses will also be investigated. These studies will answer the important question of whether R-VLP is a variant type of oncorna virus or whether it is a new type of oncogenic virus. During the progress of this research, techniques and reagents will be developed which may be used for detection of R-VLP in normal and tumor cells of animals and man.